1. Field of the Invention
This invention relates to a method for patterning an epitaxial substrate, a light emitting diode and a method for forming a light emitting diode.
2. Description of the Related Art
Referring to FIG. 1, a conventional light emitting diode (LED) includes an epitaxial substrate 11 and a light-emitting unit 12 formed on the epitaxial substrate 11. The light-emitting unit 12 includes a first type semiconductor layer 121 connected to the epitaxial substrate 11, a light-emitting layer 122 formed on a portion of the first type semiconductor layer 121, a second type semiconductor layer 123 formed on the light-emitting layer 122, and first and second electrodes 124, 125 that are respectively formed on a remaining portion of the first type semiconductor layer 121 and the second type semiconductor layer 123. When external power is applied to the first and second electrodes 124, 125, the light-emitting layer 122 will emit light due to photoelectric effect.
Presently, the light-emitting unit 12 is usually made of a GaN type semiconductor material having a refractive index of about 2.5, and air has a refractive index of 1. Accordingly, when light emitted from the light-emitting layer 122 travels to an interface between the light-emitting unit 12 and air, it is likely to totally reflect at the interface due to the refractive index differences between the light-emitting unit 12 and air, and thus, light will travel between the epitaxial substrate 11 and the light-emitting unit 12. Thus, the conventional LED has a relatively low light extraction rate. In order to reduce the total reflection at the interface between the light-emitting unit 12 and air, the epitaxial substrate 11 that is roughened to have different patterns has been proposed. For example, the epitaxial substrate 11 may be roughened to form an irregular structure on a surface thereof, or to form a matrix of protrusions on the surface thereof. With the patterns on the epitaxial substrate 11, the traveling direction of light may be varied when the light travels to the epitaxial substrate 11, thereby reducing the possibility of total reflection and enhancing the light extraction rate of the LED.
Referring to FIG. 2, a method for forming a light emitting device is disclosed in U.S. patent application publication no. 2010/0059789, in which an epitaxial substrate 2 is formed with a roughened surface so as to improve a light extraction rate of the light emitting device. The epitaxial substrate 2 is formed by: (1) lithographically patterning a photoresist layer on a substrate 21 to form a plurality of photoresist patterns 101 using a photomask (not shown); (2) conducting a first etching process on the substrate 21 that is uncovered by the photoresist patterns 101 to form a plurality of substrate patterns 102; (3) conducting a heat treatment of the photoresist patterns 101 and the substrate 21 so that the photoresist patterns 101 have uneven thickness and so that the substrate patterns 102 are deformed; and (4) conducting a second etching process on the photoresist patterns 101 and the substrate 21, followed by removing the photoresist patterns 101, so as to form the substrate patterns 102 including a plurality of protrusions 104 each of which has a concave portion 103 indented from a top face of the protrusion 104. With the substrate patterns 102, the light extraction rate of the light emitting device is enhanced, and an epitaxial layer formed on the epitaxial substrate 2 in a subsequent process has an improved quality.
The heat treatment is conducted to vary the thicknesses of the photoresist patterns 101 and the shapes of the substrate patterns 102. By virtue of the differing thickness of the photoresist, the substrate patterns 102 having the protrusions 104 and the concave portions 103 are thus obtained using the second etching process. In order to form the desired substrate patterns 102, the heat treatment and the second etching process should be precisely controlled, and the substrate patterns 102 are limited to a bowl-shape. Besides, since the differing thickness of the photoresist produced by heat treatment is relatively hard to be controlled, the uniformity of the substrate patterns 102 is hard to be achieved.